The present invention relates to an aircraft air conditioning system and a method of operating an aircraft air conditioning system.
So-called air-based air conditioning systems, as described for example in DE 10 2008 053 320 A1 and US 2010/101251 A1 or DE 10 2010 054 448 A1 and WO 2012/079756 A2, are usually used at present in commercial aircraft to air-condition the aircraft cabin. An aircraft air conditioning system serves to adequately cool or heat the aircraft cabin and to adequately supply fresh air into the aircraft cabin to ensure that a prescribed minimum proportion of oxygen is present in the aircraft cabin. Further, the humidity within the aircraft cabin is adjusted by means of the aircraft air conditioning system. Moreover, during flight operation of the aircraft, the aircraft air conditioning system is used to pressurize the aircraft cabin so as to maintain the pressure within the aircraft cabin, compared to the ambient pressure at the cruising altitude of the aircraft, at an elevated level.
Air-based air conditioning systems typically comprise an air conditioning unit, which is arranged, for example, in a wing root of the aircraft, and which is supplied with compressed process air that is generated by a compressor or bled off from an engine or an auxiliary power unit (APU) of the aircraft. During flight operation of the aircraft, usually engine bleed air is used so as to supply the air conditioning unit of the aircraft air conditioning system with compressed process air. During ground operation of the aircraft the air conditioning unit of the aircraft air conditioning system, however, typically is supplied with compressed process air from the auxiliary power unit of the aircraft. In the air conditioning unit, the process air, upon flowing through at least one heat exchanger as well as through various compression and expansion units, is cooled and expanded. Cooled process air exiting the air conditioning unit finally is supplied to a mixing chamber where it is mixed with recirculation air recirculated from an aircraft region to be air conditioned. The mixed air from the mixing chamber, via respective mixed air lines, is supplied to the aircraft region to be air conditioned which may be divided into a plurality of air conditioning zones.
DE 10 2010 034 831 A1 and WO 2012/022758 A1 describe an aircraft air conditioning system, wherein compressed process air that is bled off from an engine of the aircraft is cooled by transferring heat to a refrigerant circulating through a refrigerant circuit. A turbine which serves to expand the compressed process air prior to being directed into an aircraft cabin is used to drive a compressor disposed in the refrigerant circuit.